1. Field of the Invention
The present invention generally relates to quality inspection systems including automated and non-destructive quality inspection systems and, more particularly, to the detection of improperly formed solder connections in electronic devices.
2. Description of the Prior Art
The use of semiconductor devices and integrated circuits has become almost universal in electronic devices of many diverse types. In particular, at the present time, it is common to mount integrated circuits and other electronic components on circuit boards or within packages or modules by solder connections which form both mechanical supports for and electrical connections to such devices. While solder joints generally perform both of these functions with relatively high reliability, structural support can often be achieved while electrical continuity is not. In such a case, discovery of the improperly formed solder connection is often difficult to locate and repair. For example, so-called "cold solder joints" have required extensive testing and trouble-shooting to repair since earliest uses of printed circuit boards. However, at the present state of the art, a chip may be mounted to a lead frame, the lead frame, in turn connected to a module, the module connected to a board, etc. and an improperly formed solder joint can occur at any level of a hierarchy of structures which may be employed.
While techniques of increasing the reliability of solder connection formation have become highly developed in recent years, the miniaturization of electronic circuitry has made the detection of improperly formed solder joints much more difficult. Increased functionality has greatly multiplied the number of connections made in a single device, board or module which renders testing of each connection impractical if not impossible. At the same time, the number of connections to each chip require geometries which complicate other testing or inspection procedures. For example, some integrated circuit chips are attached to a lamina or laminated carrier of a so-called multi-layer module by forming solder connections between congruent arrays of pads on the respective surfaces thereof. Thereafter, the chip, itself, prevents visual inspection of the solder connections and complicates use of any other known inspection technique.
Further, the geometries of connections themselves may defeat some testing and/or inspection procedures. For example, in so-called quad flat pack (QFP) integrated circuit package structures, connection leads extend laterally from the four sides of the package to overlap other conductors which generally extend inwardly toward the location at which the integrated circuit is to be mounted. The overlapping portions of these respective conductors are then soldered by flowing solder around and between the overlapped portions of the connection leads.
In such a case, the number of connections made will provide a good structural support even if a substantial number provide no electrical continuity or even mechanical contact. Conversely, since overlapped portions of connection leads may be held in contact, electrical continuity may exist even where a solder connection is improperly formed and which may open when the device, board or module is placed in service. Such a flaw is generally referred to as a "pressure contact open" which may be regarded as a solder connection similar to a cold solder joint where solder is present but does not form a continuous connection between the soldered elements (e.g. having a crack or inclusion) due to insufficient temperature, dewetting, loss of flux, contaminated leads or pads, separation or noncoplanarity of leads and the like but where some degree of electrical continuity remains due to compression applied in some mechanical manner to the connection. Known techniques for detection of subsurface flaws such as ultrasonic echo, x-ray radiography and even electrical continuity testing cannot detect such a pressure contact open since there is no gap between the connection leads when the joint is under pressure and electrical continuity is generally present. Additionally, in such a structure, the act of contacting a portion of the circuitry with a test probe may cause electrical continuity where it would not otherwise be present due to a bent connection lead, lead frame warpage, cold solder, inclusions of flux or contamination and the like.
Acoustic emission testing of structures is also generally known and is an important technique in metallurgical research and testing. Acoustic emission testing is, perhaps, the technique of choice for monitoring the growth of a defect in a stressed body of material, usually in large structures such as cranes, pipelines, bridges and nuclear reactors and the like which have some particularly critical structural components. Such techniques however, are often complicated by the fact that multiple acoustic sensors are required for triangulation to discover the location of the defect or defects. U.S. Pat. No. 5,010,503 to Paton et al. is exemplary of a multi-channel triangulation system and is principally directed to reduction of the amount of data which must be processed in order to locate a defect. However, for all its complexity, this technique of acoustic emission testing requires stressing of the material being tested to stimulate propagation of the crack or other defect (which is inappropriate to electronic circuit testing) and is also not effective or reliable to detect pressure contact opens since there is no gap in a pressure contact open and no further propagation of a pressure contact open is likely (e.g. the pressure contact open is fully formed during the soldering process), even under some level of stress.
Because of the lack of a reliable technique for detecting pressure contact opens, such improperly formed solder connections have become a leading cause of failures of electronic devices including digital data processors and computer systems which have been placed in service even though the frequency of production of pressure contact opens is generally quite low. The determination of a precise location of a pressure contact open is, accordingly, often of secondary importance to the detection of a device which is otherwise tested to be fully operational but which may fail after being placed in service. Further, current soldering techniques often provide for the concurrent reflowing of many solder preforms so that all connections to one or more chips may be simultaneously formed or reformed; reducing need to resolve the location of a pressure contact open.
Therefore, a need exists for a technique of screening electronic devices, boards and modules prior to shipment or assembly which is highly effective to detect the presence of pressure contact opens. This need is so great at the present time that optical inspection of individual connections by trained personnel to determine the shape of solder connections or in connection with individual probing of connections is currently the technique of choice in the manufacture of components of high reliability and quality. Even then, the number of solder connections is often so great that inspection of all solder connections in the component is a practical impossibility.